Some magnetic speed sensors are configured to measure the speed of a magnetic tooth wheel. Such speed sensors typically include an integrated circuit with a plurality of magnetic sensor elements, such as Hall sensor elements or giant magneto resistance (GMR) sensor elements. A permanent magnet provides a bias magnetic field to the sensor elements. As the wheel is rotated, the teeth of the wheel pass in front of the sensor and generate a small field variation, which is detected by the integrated circuit. The detected field contains information about the angular position and rotational speed of the wheel.
GMR sensors were first manufactured in the 1980's. They are distinguished by their high sensitivity of their electrical resistance to the orientation of an external magnetic field. The GMR effect takes place in a limited range along one axis of the magnetic field. This range is referred to as the anisotropic range. In the anisotropic range, the sensor has a high sensitivity (resistance change versus magnetic field change). In some applications, such as a magnetic speed sensor with a back bias magnet for measuring the speed of a magnetic tooth wheel, a small misplacement or inclination of the back bias magnet with respect to a GMR element of the sensor can drive the working point of the GMR element into saturation. The back bias magnet creates an offset in the working point of the sensor from an optimal point near the center of the anisotropic range, to the saturation region. As a result, no signal or a signal of reduced quality is generated, thereby lowering the sensor performance. For gradiometric sensors that operate on a differential principle, the offset problem becomes worse as the sensor elements are located in two different positions.